I don’t really think the Necron would have been a great threat to humanity if the Horus Heresy had never happened. Necrons are only truly vulnerable to one thing, and that’s psychic powers. Were the Emperor still fighting alongside His troops, I think the Necrons would be in serious trouble. Plus, in the time since the Heresy the Emperor would also have finished whatever project He was working on when the Heresy interrupted Him; personally, I think the fluff points towards Him attempting to find a way for humanity to reach the goal the Old Ones intended for us and become the ultimate psychic race in the galaxy. If the Emperor had actually managed to do that in the ten thousand years since the birth of the Imperium, the Necrons wouldn’t stand a chance. We were always intended by the Old Ones to be the ultimate biological anti-Necron weapon, after all.
As for how the rest of the galaxy would look, I agree that most of it would be part of the human Imperium. Chaos would still exist, the Eye of Terror would still be there, and people would still be tempted and turn heretics, but Chaos would have nowhere nearly as much power as it does now. The Eldar might still be around, as would the Orks since they’re simply impossible to get rid off, but the Dark Eldar probably wouldn’t, nor the Tau. The Imperium would have evolved in very different ways, their technology boosted not only by the Mechanicus findings of STCs but also the knowledge held by the Emperor Himself, the only survivor from the Dark Age of Technology when nothing was impossible to humanity. We might as a species have reached our potential for psychic abilities and like the Eldar have come to depend on these powers for our technology and constructions.
Really, it’s impossible to tell what would have happened, but I think it’s fair to say that the Horus Heresy really crippled the Imperium. Without it, humanity could have come a very long way indeed.
~Grephaun.